1. Field of the Invention
The present invention relates to an optical information recording/reproducing apparatus for recording and/or reproducing information on and/or from an optical memory medium such as an optical disk of an optical card, using an optical head and, more particularly, to a digital servo device for performing auto-focus control and auto-tracking control of the optical head in an apparatus of this type.
2. Related Background Art
Tracks are concentrically or spirally formed on an optical disk, and the optical disk is divided into a plurality of sectors in terms of recording of variable length data or an increase in access speed of an optical head. The optical disk is subjected to optical recording/reproduction in units of sectors.
In order to cause the optical head to access each sector or track, the optical head (mainly, an optical system) must be servo-controlled in so-called focusing and tracking directions. A conventional servo control can be made in an analog or digital manner. In particular, in recent years, digital servo control has received a great deal of attention since it can stably operate a servo system. FIG. 1 shows an example of a digital servo control device.
A digital servo device of a conventional optical information recording/reproducing apparatus will be described below with reference to FIG. 1.
The servo device shown in FIG. 1 includes an optical disk 1 as a recording medium, an optical system 2 for an optical head, a tracking error detector 3 for detecting a tracking error signal based on an output from the optical system 2, and a focus error detector 4 for detecting a focus error signal on the basis of an output from the optical system 2. The tracking error detector 3 and the focus error detector 4 can adopt a known detection method in, e.g., an optical disk recording apparatus. The servo device also includes A/D converters 5 for A/D-converting error signals output from the error detectors 3 and 4 into digital signals, a digital signal process circuit 14, D/A converters 11 for converting the digital signals from the signal process circuit 14 into analog signals, and tracking and focus actuators 12 and 13 for driving the optical head 2 in predetermined directions.
The operation of the digital servo device in the optical information recording/reproducing apparatus with the above arrangement will be described below.
When information is to be recorded on or reproduced from the optical disk 1 by the optical system 2, the optical system 2 must be controlled by the focus and tracking actuators 13 and 12. The digital signal process circuit 14 obtains, using predetermined equations, control amounts necessary for accurately controlling the optical system 2 to be in a desired position in accordance with tracking and focus error signals output from the optical system 2. The actuators 12 and 13 are operated by the control amounts obtained by the digital signal process circuit 14.
Japanese Patent Laid-Open No. 63-121136 discloses the same arrangement as the conventional digital servo device described above. This prior art invention also discloses that control parameters (coefficients) corresponding to various conditions (in focusing and tracking modes)and prestored in a memory are read out, as needed, and calculations are performed using the readout parameters so that the frequency characteristics of a servo system optimally and at all times matches with actuators.
However, in the conventional digital servo device described above, a basic control process (equations) is permanently set (programmed) in the digital signal process circuit. (In the arrangement disclosed in Japanese Patent Laid-Open No. 63-121136, the basic control processes, i.e., equations, are permanently set in a DSP 38. Only coefficients of the equations can be changed according to the conditions. For example, in an equation y=a.sub.n x.sup.2 +b.sub.n x+c.sub.n, only the coefficient portions a.sub.n, b.sub.n, and c.sub.n can be changed.) More specifically, one control process is caused to have-redundancy in correspondence with all possible conditions. When an optical disk having different reflectance characteristics is used, or when an optical disk having a different track intervals or sector formats is used, the digital signal process circuit shown in FIG. 1 must use different control processes, or the actuators cannot be properly controlled. It is impossible to cope with various conditions by only the coefficients of the equations.
Some host machines (not shown) for controlling an optical disk device have different minimum recording lengths for the optical disks, In order to achieve optimal tracking, the control process used in the digital signal process circuit 14 shown in FIG. 1 must be changed. As described above, however, since the control process cannot be changed, the actuators cannot be properly controlled.
The control process is left unchanged between focusing and/or tracking servo is led in (or set up) before an operation is started (transient operation) and when a servo loop is closed after the servo is led in (steady operation) although these operation states require different control processes. As a result, optimal servo control cannot be performed in these operation states.
Furthermore, if the same control process is employed in both the focusing and tracking servo operations, optimal servo control is disturbed in these operations.